Praseodymium doped waveguide lasers

ABSTRACT

The invention is based on the discovery that under certain conditions lasing at room temperature can be achieved in a Pr 3+  -doped fluorozirconate fiber pumped in the infrared at 1.01 μm and 835 nm, the lasing taking place in the blue (491 nm), green (520 nm), orange (605 nm) and red (635 nm and 715 nm). One laser comprises a length of Pr 3+  -doped potical waveguide such as a fiber, and means for exciting the Pr 3+   ions to an energy level in the band ( 3  P 2 ,  1  I 6 ,  3  P 1 ,  3  P 0 ), in which the Pr 3+  concentration is in the range substantially 50 ppm to substantially 10,000 ppm (by weight). The Pr 3+   concentration is preferably in the range substantially 200 ppm to substantially 2,000 ppm (by weight). The optical fiber is preferably a fluorozirconate fiber doped at the foregoing concentrations with Pr 3+   ions. The fiber preferably comprises a doped core clad with a further glass. The excitation means is preferably arranged to excite the Pr 3+   ions from the  3  H 4  level, and this is preferably achieved by upconversion by way of the  1  G 4  level, but excitation may be achieved by transfer of energy from a co-dopant.

This invention relates to lasers and particularly to lasers forproducing visible radiation.

There is currently a great deal of interest in the development of simpleand compact sources of coherent visible radiation. Two promisingtechniques for the development of such sources are second harmonicgeneration, either directly of the output from laser diodes 1! or of theoutput from a laser diode-pumped laser 2!, and upconversion lasing wherethe energy from two or more photons from a pump source are absorbed by asingle ion which subsequently emits a single higher energy photon.

A variety of upconversion laser systems based on both multi ion 3! andsingle ion 4! processes have been demonstrated. This technique has theadvantage of simplicity, in that no stabilised resonant cavity isneeded, but also the disadvantage that in most cases the efficiency ofupconversion is found to be strongly temperature dependent so thatcooling to liquid nitrogen (or lower) temperatures is required. However,Allain et al 5! have recently demonstrated an efficient Ho³⁺ dopedfluorozirconate fibre laser operating in the green at room temperaturewith red krypton laser pumping.

Efforts are being made to develop a blue laser for which a variety ofuses are envisaged, including use in a compact disc player where thecorrespondingly small spot size could provide an increase in storagecapacity.

Whilst the Allain et al 5! laser does not produce blue light, that laserdemonstrated the great benefit of using the fibre geometry where thesmall core diameter allowed the high intensities required for efficientupconversion to be maintained over a long interaction length. Allain etal 6! have also previously reported on Pr³⁺ -doped fluorozirconate fibrelasers operating in the orange and red when pumped with an argon laseroperating at 476.5 nm.

The present invention is based on our discovery that under certainconditions we have achieved lasing at room temperature in a Pr³⁺ -dopedfluorozirconate fibre pumped in the infrared at 1.01 μm and 835 nm, thelasing taking place in the blue (491 nm), green (520 nm), orange (605nm) and red (635 nm and 715 nm).

One great attraction of pumping at infrared wavelengths is that highpower laser diodes are available and so it may be possible to constructefficient, high power, all-solid-state blue green and red sources basedon upconversion in Pr³⁺ -doped fibres. Such sources may be expected tofind applications in areas such as optical data storage, underseacommunications and projection televisions.

According to one aspect of the invention a laser comprises a length ofPr³⁺ -doped optical waveguide, and means for exciting the Pr³⁺ ions toan energy level in the band (³ P₂, ¹ I₆, ³ P₁, ³ P₀), in which the Pr³⁺concentration is in the range substantially 50 ppm to substantially10,000 ppm (by weight).

The waveguide is preferably in the form of a fibre.

The Pr³⁺ concentration is preferably in the range substantially 200 ppmto substantially 2,000 ppm (by weight).

The optical fibre is preferably a fluorozirconate fibre doped at theforegoing concentrations with Pr³⁺ ions.

The fibre preferably comprises a doped core clad with a further glass.

The numerical aperture of the clad fibre is preferably in the rangesubstantially 0.1 to substantially 0.5, and is typically 0.15.

The core diameter of the fibre is preferably in the range substantially1 μm to 5 μm.

The excitation means is preferably arranged to excite the Pr³⁺ ions fromthe ³ H₄ level, and this is preferably achieved by upconversion by wayof the ¹ G₄ level, but excitation may be achieved by transfer of energyfrom a co-dopant, preferably another rare earth ion.

Excitation by way of the ¹ G₄ level is preferred because the energy gapfrom ³ H₄ to ¹ G₄ corresponds to 1.01 μm and that from the ¹ G₄ to the ¹I₆, ³ P₁ common level in said band is 835 nm, both in the infrared rangefor which powerful infrared sources are available. The excitation meanspreferably comprises first excitation means for exciting the Pr³⁺ ionsfrom ³ H₄ to ¹ G₄, and second excitation means for exciting the ionsfrom ¹ G₄ to the ¹ I₆, ³ P₁, ³ P₀, ³ P₂ levels.

It may, however, at relatively high concentrations of Pr³⁺ ions bepossible to populate the ¹ G₄ level by an avalanche process (photonavalanche upconversion) such that the excitation means need provide asingle wavelength excitation from the ¹ G₄ level to said band.

Instead of cladding the fibre core with a single layer of glass, twocladding layers of different refractive indexes might be employed, theradially outer cladding layer being of lower refractive index. One ofthe lasers used as the pumping agency could then be directed into theinner layer of the cladding where lasing at a suitable first infraredwavelength could be achieved and which pumps the ions in the core. Asecond suitable infrared laser wavelength is also launched into thecore, thus achieving the upconversion pumping.

These two fibres would share a common resonator, such that the infraredemission from one fibre provides one pump for the Pr³⁺ -doped fibre, andthe other infrared pump is launched externally.

According to a second aspect of the invention a laser comprises a Pr³⁺-doped optical waveguide means, first excitation means for exciting thePr³⁺ ions from the ³ H₄ level to the ¹ G₄ level, second excitation meansfor exciting the ions from the ¹ G₄ level to a level in the band (³ P₂,¹ I₆, ³ P₁, ³ P₀), and reflection means so arranged in relation to thewaveguide that lasing is produced for at least two visible wavelengths.

The waveguide means is preferably an optical fibre.

Preferably the reflection means is arranged such that lasing is producedat at least three visible wavelengths, one of which is the bluetransition 491 nm from ³ P₀ to ³ H₄.

A third aspect of the invention comprises operating a laser inaccordance with either the first or second aspects of the invention atsubstantially room temperature (eg 20° C.) to produce at least onevisible wavelength.

According to a fourth aspect of the invention a laser comprises a Pr³⁺-doped optical waveguide means, and infrared diode laser means arrangedto excite the Pr³⁺ ions to the bands (³ P₂, ¹ I₆, ³ P₁, ³ P₀).

A fifth aspect of the invention comprises a Pr³⁺ -doped opticalwaveguide means, providing high gain amplification, at visiblewavelengths, suitable for amplifying visible diode lasers and othervisible sources.

Some experiments to demonstrate the feasibility of a laser in accordancewith the invention will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 is an energy level diagram for Pr³⁺ -doped ZBLANP glass showingpumping scheme and laser transitions,

FIG. 2 shows output power at 635 nm as a function of pump power for aPr³⁺ -doped ZBLANP fibre laser (fibre length=10 m),

FIG. 3 shows output power at 605 nm as a function of pump power for aPr³⁺ -doped ZBLANP fibre laser (fibre length=1.2 m).

EXPERIMENTS

The fibre used for the experiments described here had a ZBLANP coredoped with Pr³⁺ ions at a concentration of 560 ppm (by weight) and acladding of ZBLAN glass.

ZBLANP is a zirconium barium lanthanum aluminium sodium lead fluoride.

ZBLAN is a zirconium barium lanthanum aluminium sodium fluoride.

The core diameter of the fibre was 4.6 μm and the numerical aperture0.15, implying a cut-off wavelength for the LP₁₁ mode of 0.9 μm. Anenergy level diagram for the Pr³⁺ ion in a ZBLANP host glass is shown inFIG. 1. A Ti:sapphire laser tuned to 1.01 μm was used to excite Pr³⁺ions from the ³ H₄ ground state to the ¹ G₄ multiplet. A secondTi:sapphire laser tuned to 835 nm was used to provide excitation fromthe ¹ G₄ multiplet to the thermally coupled ³ P₁, ¹ I₆ and ³ P₀ levels.

Initial experiments on the red laser transition were carried out with afibre length of approximately 10 meters. Pump light from bothTi:sapphire lasers was combined using a polarization rotator andbeamsplitter. This light was launched co-propagating into the fibre by a×20 microscope objective at an efficiency of ≈30-40% for each pump beam.At the launch end the fibre was butted against a dielectric mirrorof >99% reflectivity from 600 nm to 640 nm and ≈80% transmission at bothpump wavelengths. The laser cavity was completed by the ≈4% Fresnelreflection arising from the fibre/air interface. The pump power from theTi:sapphire laser operating at 835 nm was set at 700 mW incident on thelaunch objective. The laser power at 635 nm (³ P₀ -³ F₂ transition) wasthen monitored as a function of the power from the Ti:sapphire laseroperating at 1.01 μm and these results are shown in FIG. 2. The slopeefficiency with respect to incident 1.01 μm pump power was ≈9.6%. Themaximum power extracted from this laser was ≈185 mW. This data point wasobtained by tuning the pump wavelength to 995 nm where 2 W of pump powerwere available and represents an overall power conversion efficiency ofnearly 7% for infrared light to the red. The pump power was then set to1 W at 1.01 μm and the red output power at 635 nm measured as a functionof 835 nm pump power. This data set is also shown in FIG. 2. The slopeefficiency with respect to incident 835 nm pump power was ≈14%. Somesaturation of the 635 nm output power with respect to 835 nm pump poweris evident. It is thought that this may possibly arise from thesaturation of the 835 nm excited state absorption, although it may alsobe due to the 835 nm pump beam moving (and hence launch efficiencychanging) as the power was attenuated. Significant improvements to theseresults may be expected when using a fibre of lower background loss thanthe current value of 0.3 dB/m. Pr³⁺ -doped ZBLAN fibres with losses of≈0.1 dB/m have previously been fabricated 7!. The red transition at 635nm was the only one on which laser oscillation could be obtained whenusing the 10 m length of fibre.

Since it was clear that by no means all of the 10 m length of fibre wassignificantly pumped further experiments were then carried out on afibre length of ≈1.2 m. With this length of fibre the 635 nm transitionwas below threshold unless added feedback was provided and so it waspossible to investigate other laser transitions. Using a cavitycontaining two mirrors of >99% reflectivity in the red butted againstthe fibre, incident pump power thresholds as low as 40 mW at 1.01 μmwith 10 mW at 835 nm and 40 mW at 835 nm with 20 mW at 1.01 μm wereobtained. This fibre length is much shorter than the optimum since theground state absorption at 1.01 μm is <2 dB/m for this fibre and so alonger fibre length would allow more pump power to be absorbed.

The mirror at the output end of the cavity was then changed to one of≈40% reflectivity at 605 nm and ≈20% reflectivity at 635 nm. With thiscavity laser oscillation was observed at 605 nm on the ³ P₀ -³ H₆transition. The 1.01 μm pump power was set to 1 W and the 605 nm outputpower measured as a function of the 835 nm output power and the resultsare shown in FIG. 3. For low values of 835 nm pump power the slopeefficiency is approximately 7% with respect to incident pump power. Thesaturation of the 605 nm output power with respect to 835 nm pump poweris thought to arise from the saturation of the 835 nm excited stateabsorption. The maximum 605 nm power extracted was approximately 30 mW.An improved performance should be possible with a longer fibre lengthwhere there would greater absorption at both pump wavelengths. The 835nm power was then set to 600 mW and the 605 nm output power measured asa function of 1.01 μm power. These results are also shown in FIG. 3,where the slope efficiency with respect to 1.01 μm pump power is ≈3.3%.

Both mirrors were then changed to mirrors of >99% reflectivity in thegreen. With a cavity completed with the mirrors laser oscillation wasobserved at 520 nm on the ³ P₁, ¹ I₆ -³ H₅ transition. A threshold of≈160 mW of each pump wavelength was measured. Since the output couplingwas <1%, the extracted output power was only of the order of 1 mW. Forhigh pump powers simultaneous lasing in the green and red was observed.Clearly significant improvements in performance should be possible witha cavity using an optimised fibre length and an output coupler of highertransmission.

Laser oscillation has also been observed in the blue at 491 nm on the ³P₀ -³ H₄ three-level transition when completing the cavity with two highreflectors at this wavelength. The lowest threshold recorded was ≈200 mWof 835 nm pump power and 280 mW at 1.01 μm. Again, the extracted powerswere of the order of 1 mW because of the low transmission of the outputcoupler. For high pump powers simultaneous lasing at 635 nm occurred.

DISCUSSION

We have demonstrated continuous-wave room temperature infrared-pumpedupconversion lasers based on Pr³⁺ -doped fluorozirconate fibre whichoperate at blue, green, orange and red wavelengths. We believe thatthese are the first reported CW room temperature infrared pumped visiblelasers. There is clearly plenty of scope for improving on theperformance that we have obtained to date. By using a smaller core fibrewith a cut-off wavelength of ≈450 nm it should be possible to reduce thepump powers required at each wavelength by up to a factor of four sincethe intensity scales inversely with the core area. This will reduce thethreshold powers required for laser oscillation in the red to a levelwell within reach of that available from laser diodes. In addition toimprovements resulting from the reduction in core diameter, furthersignificant improvements should result from using a longer fibre lengththan the 1.2 m used for demonstrating laser oscillation in the blue,green and orange. Additionally, any pump light which was not absorbed ina single pass through the fibre could be fed back to be absorbed on asecond pass. It is, therefore, not unreasonable to expect the thresholdsfor lasing on these transitions also to come down to a level availablefrom semiconductor diode lasers. We believe that the results containedin this letter represent a significant step towards the realization ofpractical all-solid-state blue green and red upconversion lasers. Suchdevices have the attraction of cheapness and simplicity and could beexpected to find numerous applications in a wide variety of fields.

In order to obtain a suitable balancing between outputs of blue, greenand red light it may be necessary to provide means for absorbing orsuppressing production of some of the red light, and such an absorptionmeans preferably takes the form of a suitable co-dopant which can bearranged either in the core or in the cladding.

REFERENCES

1! Kozlovsky, W. J., Lenth, W., Latta, E. E., Moser, A. and Bona, G. L.:"Generation of 41 mW of blue radiation by frequency doubling of a GaAlAsdiode laser", Appl. Phys. Lett., 1990, 56, pp.2291-2292

2! Risk, W. P., Pon, R., and Lenth, W.: "Diode laser pumped blue-lightsource at 473 nm using intracavity frequency doubling of a 946 nm Nd:YAGlaser", Appl. Phys. Lett., 1989, 54, pp.1625-1627

3! Hebert, T., Wannemacher, R., Lenth, W. and Macfarlane, R. M.: "Blueand green cw upconversion lasing in Er:YLF₄ ", Appl. Phys. Lett., 1990,57, pp.1727-1729

4! Allain, J. Y., Monerie, M., and Poignant, H.: "Blue upconversionfluorozirconate fibre laser", Electron. Lett., 1990, 26, pp.166-168

5! Allain, J. Y., Monerie, M., and Poignant, H.: "Room temperature CWtunable green upconversion holmium fibre laser", Electron. Lett., 1990,26, pp.261-263

6! Allain, J. Y., Monerie, M., and Poignant, H.: "Tunable CW lasingaround 610, 635, 695, 715, 885 and 910 nm in praseodymium-dopedfluorozirconate fibre", Electron. Lett., 1991, 127, pp.189-191

7! Carter, S. F., Szebesta, D., Davey, S. T., Wyatt, R., Brierley, M. C.and France, P. W.: "Amplification at 1.3 μm in a Pr³⁺ -doped single-modefluorozirconate fibre". Electron. Lett., 1991, 27, pp.628-629

We claim:
 1. An optical fibre laser for upconversion of infrared energy,comprising:first means for generating infrared radiation that has afirst wavelength of approximately 1010 nanometers; second means forgenerating infrared radiation that has a second wavelength ofapproximately 835 nanometers; Pr³⁺ -doped optical fibre in which theconcentration of Pr³⁺ is in the range of substantially 50 ppm to 10,000ppm for absorbing said first and second wavelengths to generate visibleradiation that has a wavelength in the range of approximately 491 to 635nanometers; and reflection means, coupled to said first and second meansfor generating infrared radiation and said Pr³⁺ -doped optical fibre,for generating optical feedback at said generated visible radiationwavelength to cause lasing of said generated visible radiation.
 2. Thelaser as set forth in claim 1, wherein said first wavelength radiationraises the energy level of at least some of said Pr³⁺ ions from the ³ H₄state to the ¹ G₄ state, and said second wavelength radiation raises theenergy level of at least some of said Pr³⁺ ions in the ¹ G₄ state to astate selected from the group of energy levels comprising the ¹ I₆, ³P₁, ³ P₀ and ³ P₂ levels.
 3. The laser as set forth in claim 2, whereinsaid reflection means generates optical feedback at a wavelength ofapproximately 635 nanometers to cause lasing at a wavelength ofapproximately 635 nanometers caused by resonant enhancement of radiativedecay of said Pr³⁺ ions at the ³ P₀ level to the ³ F₂ level.
 4. Thelaser as set forth in claim 2, wherein said reflection means generatesoptical feedback at a wavelength of approximately 605 nanometers tocause lasing at a wavelength of approximately 605 nanometers caused byresonant enhancement of radiative decay of said Pr³⁺ ions at the ³ P₀level to the ³ H₆ level.
 5. The laser as set forth in claim 2, whereinsaid reflection means generates optical feedback at a wavelength ofapproximately 520 nanometers to cause lasing at a wavelength ofapproximately 520 nanometers caused by resonant enhancement of radiativedecay of said Pr³⁺ ions at the ³ P₀ and ¹ I₆ levels to the ³ H₅ level.6. The laser as set forth in claim 2, wherein said reflection meansgenerates optical feedback at a wavelength of approximately 491nanometers to cause lasing at a wavelength of approximately 491nanometers caused by resonant enhancement of radiative decay of saidPr³⁺ ions at the ³ P₀ level to the ³ H₄ level.